1. Field of the Invention
The present invention relates to a phase lock oscillator comprising a voltage controlled oscillator whose oscillation frequency changes according to a control voltage and a communication equipment with the phase lock oscillator mounted therein.
2. Description of the Related Art
There have been seriously demanded downsizing, lightening, and reduction in power consumption for terminal equipment of a portable type which can provide a communication service to subscribers of a mobile communication system, so that the terminal equipment incorporates a frequency synthesizer which can realize a smooth access to a desired radio channel in a process of channel control performed in cooperation with a radio base station.
FIG. 11 is a diagram showing a structural example of a conventional frequency synthesizer.
In the drawing, a first input of a phase comparator 71 is given a reference signal with a known frequency and an output of the phase comparator 71 is connected to one input of a voltage controlled oscillator (VCO) 73 via a loop filter 72. An output of the voltage controlled oscillator 73 is connected to an input of a circuit (not shown) to which a local-frequency signal generated by the voltage controlled oscillator 73 is to be given and an input of a variable frequency divider 74. An output of the variable frequency divider 74 is connected to a second input of the phase comparator 71. Another input of the voltage controlled oscillator 73, a third input of the phase comparator 71, and a control input of the variable frequency divider 74 are given control signals by a controlling part which is not shown.
The voltage controlled oscillator 73 is composed of the following components:    a variable-capacitance diode 81 in which a cathode is directly connected to an output of the loop filter 72 and an anode is grounded;    capacitors 82-1, 82-2 which are connected in cascade to the output of the loop filter 72, and a negative resistor 83 (supposed to be composed of an amplifier and so on which satisfy an oscillating condition of the voltage controlled oscillator 73 in order to simplify the explanation here);    capacitors 84-1 to 84-4 with one-side ends thereof directly connected to connecting points of capacitors 82-1, 82-2;    an inductor 85 with one end thereof directly connected to a connecting point of the capacitors 82-1, 82-2 and another end grounded;    switches 86-1 to 86-4 with one-side contacts thereof directly and individually connected to other ends of the capacitors 84-1 to 84-4 and other contacts grounded; and    a decoder 87 in which an input thereof is given the control signal and which has four output terminals directly and individually connected to control terminals of the switches 86-1 to 86-4.
In the frequency synthesizer as structured above, either one of the switches 86-1 to 86-4 provided in the voltage controlled oscillator 73 is set to on-position in response to a command which is given by the decoder 87 in response to values of the control signal (supposed to be either one of four values corresponding to the switches 86-1 to 86-4 respectively in order to simplify the explanation here).
As shown in FIG. 12 (1) to (4), the voltage controlled oscillator 73 generates a local-frequency signal with an oscillation frequency f which corresponds to a combination of capacitance of the capacitors whose other ends are grounded via the switches set to on-position, out of the capacitors 84-1 to 84-4, and a capacitance of the variable-capacitance diode 81 which increases/decreases according to a control voltage given by the loop filter 72.
Meanwhile, the controlling part forcibly initializes a state of a phase-locked loop from the output of the voltage controlled oscillator 73 to the input of the voltage controlled oscillator 73 via the variable frequency divider 74, the phase comparator 71, and the loop filter 72 at the start to set an instantaneous value of the control voltage mentioned above at an initial value (supposed to correspond to a mid-point of a variable range of the control voltage in order to simplify the explanation here). Incidentally, the controlling part gives a predetermined constant appropriate for a prescribed frequency, which is described later, to the variable frequency divider 74 as a division ratio.
The controlling part also varies values of the above-described control signals step by step at a predetermined cycle to compare a frequency of the local-frequency signal generated by the voltage controlled oscillator 73 according to these values and the control voltage (=the initial value) with the prescribed frequency and specifies a value (hereinafter called a ‘rough tuning value’), out of these values, at which the local-frequency signal with the prescribed frequency can be generated within the variable range of the control voltage (FIG. 13 (1)).
Furthermore, the controlling part maintains the value of the control signal at this rough tuning value and releases the initialization of the phase-locked loop described above (FIG. 13 (2)).
Therefore, even when a band of the local-frequency signal to be generated by the voltage controlled oscillator 73 is broad, the phase-locked loop can shift to a lock state (FIG. 13 (3), (4)) as long as a frequency of the local-frequency signal substantially corresponds to a middle of a partial band corresponding to the ‘rough tuning value’, out of a plurality of partial bands which divide the band.
Note that the process in which the phase-locked loop shifts to the lock state in this way is called simply a ‘self-tuning’ hereinafter.
In the conventional example described above, the variable range of the capacitance of the variable-capacitance diode 81 is neither always set at a value large enough to vary the frequency of the local-frequency signal over a prescribed broad band without changing the value of the control signal, nor is a quality factor Q of the inductor 85 at a sufficiently large value.
Therefore, it has been difficult to realize a frequency synthesizer with a broad band unless a control system in which the complication of a hardware structure thereof is tolerated and the partial bands can be switched at a high rate is applied thereto.
However, in general, it is difficult to apply such a structure to terminal equipment of a portable type in which cost reduction, downsizing, and lightening are severely demanded as well as reduction in power consumption.
Furthermore, in recent years, the integration of a radio part which is to be mounted in terminal equipment of a portable type is strongly demanded with the aim of realizing the above demands, and as a result, the variable range of the capacitance of the variable-capacitance diode 81 and a feasible value of the quality factor Q of the inductor 85 are actually limited to small values as well.
Therefore, the lock state mentioned above is not always maintained when temperature, a power-supply voltage, and other environment conditions change, which may possibly lower service quality and transmission quality since the maintenance of a radio transmission channel is prevented.
This degeneration of service quality and transmission quality can be prevented when the self-tuning is automatically retried at a predetermined frequency in a channel control process.
However, in most mobile communication systems to which a multiple access system other than a TDMA system is applied, a terminal in which a call (including a call realizing a hand off occurs, has to continuously transmit/receive some information to/from a base station unless the call is extinct.
Consequently, it has been difficult to actually employ the configuration in which the self-tuning is repeatedly retried at a predetermined frequency.